Conventional guns and projectile launching weapons utilize the burning of chemical propellants to achieve high projectile velocities. In recent years there has been a renewed interest in projectile launchers which utilize electromagnetic energy. Such electromagnetic launchers may find application in space launched weaponry and impact fusion as well as in more conventional ordnance. Generally speaking, electromagnetic launchers promise greater projectile velocities than launchers utilizing chemical propellants.
In electromagnetic launchers (also called railguns) large current impulses are introduced into current-carrying rail conductors to accelerate a projectile (often termed an armature).
An example of a novel railgun design together with a discussion of its operational principles and prior art is contained in applicant's co-pending application, entitled "Electromagnetic Injector/Railgun," Serial No. 910,915, filed September 22, 1986, now abandoned the entire disclosure of which is hereby incorporated by reference.
In general, in railgun applications, projectile velocity increases with increasing current. However, the magnitude of the current cannot be increased without limit due to joule heating of the rails, together with radiative heating of the railgun materials by plasmas, and the structural loading on the rails created by high magnetic pressures. The first-mentioned problem, joule heating, is caused by current flowing in a conductor with finite resistance. Joule heating effects are most severe when the projectile or armature is moving at high velocity and rapidly exposing new conductor material to intense currents which do not have sufficient time to diffuse into the body of the conductor. The rate of current diffusion into the body of the conductor depends upon the resistivity of the conductor. The slower the current diffusion, the larger the material resistivity. Joule heating causes rail erosion during railgun operation. This rail erosion limits the repetition rate capability and the operating life of the railgun. Joule heating in rails is not uniform because the current density is not uniform throughout the rail cross-section. Higher current densities exist near the rail surfaces and corners. Consequently, if the resistivity of the rail material could be reduced, then high peak currents, greater efficiency, high repitition rates and less rail erosion may be achieved. For example, the present inventor has developed a small-caliber electromagnetic launcher which operates at voltages below 1,000 volts. The total system resistance is 3 milliohms. The 3 milliohm resistance consists of 1 milliohm equivalent series resistance (ESR) of a capacitor bank, 1 milliohm resistance of cables and connectors, and finally, 1 milliohm resistance in the copper rails themselves. Consequently, if the resistance of the rails could be reduced to zero, then the total system resistance would be decreased by one-third. Therefore, system current would be increased by approximately the same amount, namely one-third. At present, with 3 milliohms total resistance, the peak current achieved with the already-developed device is 150,000 amperes. If the resistance of the rails could be reduced to zero, the peak current would increase to approximately 200,000 amperes. Since the velocity achieved by the projectile is approximately a linear function of rail current, a current increase of one-third, yields nearly a one-third increase in projectile velocity.
Those concerned with improving railgun performance have consistently felt a need to reduce the resistance of the rails by inducing the superconducting state. The achievement of the superconducting state has hitherto been difficult and costly because of the very low temperatures required. A discussion of the application of superconductivity to railguns is found in C. Homan et al., "Evaluation of superconducting Augmentation of Railgun Systems," IEEE Trans. on Magnetics, Vol. 20, No. 2, 03/84.
Recent developments in the field of superconductivity have produced a large variety of new ceramic-type materials which are capable of achieving the superconducting state at critical temperatures above 77.degree. K., the boiling point of liquid nitrogen. The critical temperature is the temperature at which the material becomes superconducting. The new class of materials (termed for convenience "superconducting ceramics" herein - even for materials which are not basically ceramic in nature) have been extensively discussed in the popular press. For example, the New York Times, on March 20, 1987 reported the existence of superconducting ceramics and described the making of such materials into sheets of vinyl-like tape and washer shapes. Furthermore, Electronics in its April 2, 1987 issue on pp 49-51 reported the making of superconducting ceramics into wire shapes.
The composition and manufacture of superconducting ceramics is discussed, for example, in Physics Today, pp 17-23 April 1987 which is incorporated herein by reference. An entire class of compounds with the chemical composition R Ba.sub.2 Cu.sub.3 O.sub.9-y, where R stands for a transition material or a rarr earth ion and y is a number less than 9, preferably 2.1.+-.0.05 has demonstrated superconductive properties above 90.degree. K. This class of materials is included in the terms "superconducting ceramic" and "rare-earth doped copper oxide" as used herein. Scandium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, ytterbium, and lutetium are acceptable substitutes for R above. The crystal structure of these compounds is described as an orthorhombically distorted perovskite structure.
Some compounds are formulated substituting strontium for barium. For example, La.sub.z-x Sr.sub.x CuO.sub.4-y has exhibited superconductivity at high temperatures, as reported in Physical Review Abstracts, p. 11, vol. 18, No. 9, May 1, 1987.
Fabrication of superconducting ceramics is discussed in the above-mentioned Physics Today article. A detailed discussion of the fabrication and physical properties of a typical superconducting ceramic is also found in: R. J. Cava et al., "Bulk Superconductivity at 91.degree. K. in Single Phase Oxygen - Deficient Perovskite Ba.sub.2 Y Cu.sub.3 O.sub.9-.delta., Physical Review Letters, pp 1676-1679, 20 April vol. 58, number 16.
Another important technological development is the advent of small, relatively portable cryorefrigerators. Some small cryorefrigerators employ liquid nitrogen (with a boiling point of 77.degree. K.) and others, such as the Welch cryorefrigerator use compressed air to generate temperatures as low as 98.degree. K. (-175.degree. C.). The Welch cryorefrigerator is a compact mechanical refrigerator which utilizes compressed air to achieve low temperatures.
Another small cryorefrigerator is the Cryodyne.RTM. closed cycle helium refrigerator manufactured by CTI Cryogenics. The Cryodine.RTM. cryorefrigerators are capable of cooling to temperatures of 77.degree. K. (in some applications, according to the manufacturer, Cryodine.RTM. units are used to cool scientific equipment to 6.degree. K.). The Cryodine.RTM. cryorefrigerators utilize helium supplied through a compressor.
Despite these advances there remains a continuing need for simple railguns with low electrical resistance.